Threaded caps used to seal correspondingly threaded containers are generally known as screw caps. Screw caps for containers are ideally tightened at a predetermined torque. This w torque is selected to close the container sufficiently tightly to avoid loss, deterioration or contamination of the contents during transportation and storage. However, the cap must not be so tightened that it cannot be opened manually. Also it must not be so tightened that either the cap or the container or both are damaged. The position of the threaded cap on the corresponding threads of the container determines when the cap has been properly installed on the container. This position is determined by the number of rotations made be the cap once the threads have engaged.
Capping machines for screwing threaded caps onto containers typically have a rotatable turret around the circumference of which are multiple spindles. Each spindle is caused to rotate by the rotation of the turret or a separate drive motor. All spindles turn at the same rotational velocity. Each spindle has a clutch coupled to a capping chuck at its lower end. The capping chuck may be of the magnetic, spring or friction type.
To install a cap on a container the container is held in position. A cap is held in the chuck, which is lowered toward the container. The chuck is rotated by the spindle in a direction to tighten the cap. When the cap engages the container the rotation is continued and the cap engages the container threads. Rotation of the cap continues and the cap is tightened onto the container threads. A clutch set to slip at a selected torque prevents the cap from being over tightened. After a selected time period of cap rotation, the chuck is retracted as the spindle moves upward. The next container is then presented for capping.
Caps of different sizes and materials are installed on their corresponding containers using different torques to achieve desired tightness. Tightness of the cap is controlled generally by maintaining a constant spindle rotational velocity and adjusting the clutch. In addition to the torque setting of the clutch the rotational inertia of the chuck, where it is in contact with the cap, contributes to the final tightness. The clutch setting may be set at a selected value, but the rotational inertia varies with the rotational velocity of the spindle.
A sufficient number of spindle turns is required to achieve a selected or target torque. This number of turns is generally determined by the amount of thread engagement between the cap and the container. Too small a number of spindle turns results in insufficient thread engagement between the cap and the container. This results in insufficient tightness of the cap. Too large a number of spindle turns results in excessive slipping in the clutch, thereby resulting in less consistent torque control and less efficient cap application.
High speed operation of the capping machine results in high angular velocities of the spindle and the chuck, which may result in over-tightening of the cap. There has been no method of measuring actual application torque directly from the chuck, spindle or turret and thereby determining during the capping operation at which point in the capping cycle a selected application torque has been reached. Quality control testing must be performed to assure that application torque has not changed during a capping operation, as may occur due to calibration drift and wear in the mechanical components.
Acceptable tightness is determined by running a number of containers through the capping process, then measuring the torque required to remove the cap. This removal torque must be correlated to an application torque for setting the capping machine. A number of iterations may be required to set the proper application torque. This arbitrary calibration may vary from machine to machine. It may also be difficult to maintain uniformity between the various spindles on a turret.
It is difficult to maintain constant application torque based on the arbitrary calibration. Changes in spindle rotational velocity, temperature related changes in frictional coefficients of the cap and container, and changes in the clutch, particularly in friction clutches, during operation can cause changes in application torque. Changing any of the variables of capping machine speed, cap or container size or thread configuration, temperature or other variables, requires recalibration of the machine by a skilled operator and results in lost production time.
Capping machines of the prior art have been made to run at a constant spindle speed to help maintain constant application torque. However, cross-threading or defective threads, even with constant spindle speed, can cause a selected application torque to be reached and clutch slipping to occur as desired, but resulting in an undetected defectively capped container. Running a capping machine at constant spindle speed longer than necessary to tighten the cap results in acceptable application torque because the clutch slips. However, excessive clutch wear can occur when the clutch slips for longer than necessary.
Thus there exists a need for a method and apparatus which permits quick, efficient and convenient screw cap closing of a threaded container to a preselected tightness by threading a screw cap to a selected position onto a threaded container with a selected number of rotations with a controllable and verifiable application torque.